Present data suggests that a process of fibroblast differentiation may occur during normal development and thus may play a role in normal tissue morphogenesis. Fibroblasts from different tissues, although morphologically very similar to one another, may be distinguishable only by the tissue-specific arrays of extracellular structural proteins and protein-polysaccharides which they synthesize. Only two primary fibroblast populations from different tissues have been examined in detail, however, and these come from different organisms. In order to determine whether fibroblast differentiation occurs within a single species, careful analysis of the protein-polysaccharides and fibrous proteins synthesized under conditions of in vitro culture by isolated primary fibroblast populations obtained from different tissues of the chick will be conducted. Studies of fibroblasts as they are moved from in vivo to in vitro culture will be undertaken using corneal fibroblasts, since these cells can be obtained quickly as a pure population of pre-existing fibroblasts by tissue dissection. This fibroblast population will be analyzed for its ability to 1) display, during differentiation in vitro, specific characteristics shown only by its in vivo mesenchymal progenitor population, 2) synthesize the corneal polysaccharide, keratan sulfate and the noncorneal polysaccharide, heparan sulfate in vitro, and 3) transform into chondrocytes and osteocytes in vitro. Experiments with embryonic corneas will determine whether chondroitin 4-sulfate chains are attached to the same core protein molecules as keratan sulfate chains and will serve as a basis for experiments on the regulation of polysaccharide biosynthesis during corneal development.